RISC-V: defconfigs: Remove redundant K210 DT source

The "k210_generic" DT has been the default in Kconfig since 67d96729


("riscv: Update Canaan Kendryte K210 device tree"), so drop it from the
defconfigs to avoid diff with savedefconfig.

Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>